"You can't do a post-election vote tabulation audit in such cases because there is no independent record of the votes," Smith said. "You are checking the system against itself. It is sort of a circular argument," she said. Even a few incorrectly counted or missing votes could make all the difference in a tight election especially if it happens in a key swing state, she said.

Thad Hall, an associate professor of political science at the University of Utah, said paperless DRE systems offer a degree of auditability, but not much.

"If you vote on a paperless DRE system, there are places within the machine that record the data," Hall said. "But if I don't trust the machine, I'm not going to trust the backup electronic records," said Hall, who was one of the authors of a recent MIT/Caltech report on e-voting technologies. "Sure they are auditable. The problem is that people are not going to believe the audit record," because it is not independent of the system.

Several states, including New Jersey and Maryland, have passed legislation mandating a move to paper ballots, but budget constraints have kept them on paperless DRE systems, Hall and Smith said.

But William Kelleher, CEO of The Internet Voting Research and Education Fund, said concerns about paperless DRE systems are overblown. "Just because the DREs have a "black box" in which votes are stored and tallied, doesn't mean the machines shouldn't be trusted," Kelleher said in an email. "We trust jet planes, which are at least just as much a black box," he said.

Machines may make rare counting errors if, for example, they are in need of recalibration, he said. "But humans make far more counting errors than the computers in the DREs. Perfection is not a standard that pragmatic people expect. Our election officials have determined that when the costs and benefits are balanced, paperless DREs present an acceptable risk."